MAGNETIC STRATIGRAPHY OF THE UPPER MIOCENE (EARLY HEMPHILLIAN) RATTLESNAKE FORMATION, CENTRAL OREGON

The Rattlesnake Formation in the John Day region of central Oregon has been famous for over a century for its early Hemphillian (late Miocene) fossil mammals. The sedimentary portions of the formation consist of about 85 m of conglomerate (from which most of the fossils come) overlain by the rhyolitic Rattlesnake Ash-Flow Tuff (RAFT), then about 30 m of additional fossiliferous sandstones and siltstones. Paleomagnetic sampling was conducted on both the lower and upper sedimentary sequences in the type section in Rattlesnake Creek, and just to the south of the type section. The samples were analyzed with both thermal and alternating field demagnetization. Most samples showed a single component of remanence held mainly in magnetite, with slight goethite overprints. The basal 25 m of the section is normal in polarity, but the entire upper 100 m of the section (including the RAFT) is reversed in polarity. Based on the Ar/Ar date of 7.05 +/- 0.01 Ma on the RAFT, we correlate the type Rattlesnake section with magnetic Chrons C4n to C3Bn or C3Ar (6.8-7.5 Ma). This confirms the late early Hemphillian age of the formation, and helps to date the expansion of C4 grasslands in the late Miocene of North America at about 7.5 Ma.